1. Field of the Invention
The present invention relates to a process for preparing an orientation film for liquid crystals, and more particularly to a process for preparing an orientation film for liquid crystals for use in liquid crystal display devices.
2. Description of the Prior Art
Orientation films for liquid crystals heretofore known for use in liquid crystal display devices include those prepared from synthetic resins such as polyimide resin, polyamide resin, polyvinyl alcohol and the like.
Such an orientation film is prepared by forming a layer of synthetic resin like the one mentioned above on a liquid crystal display substrate provided with transparent electrodes and the like, and rubbing the synthetic resin layer (Okano et al., "Orientation of Liquid Crystal Molecules by Surface Treatment," Liquid Crystal Application Book, pp. 56-57, Baifukan, published 1985). The resin layer is rubbed using a device which includes a rubbing roller covered with a flocked fabric of synthetic fiber. This device will be described below with reference to FIGS. 3 (a), (b) and (c).
These drawings show a rubbing roller 1, a roller shaft 2, brackets 3 for supporting the roller shaft 2, a substrate 4 formed with a synthetic resin layer, a rubbing stage 5 and a stage table 6. The rubbing roller 1, which has a flocked fabric affixed to its surface, is rotated and is also moved relative to the stage table 6 by unillustrated drive means to cause the roller 1 to rub the entire surface of the resin layer on the substrate 4.
The synthetic resin layer is thus rubbed in order to orient liquid crystal molecules orderly for fabricating a liquid crystal display device. The molecular chains at the surface of the resin layer are arranged in a specified direction by this treatment, affording an orientation film for the liquid crystals.
When the molecular chains are to be orderly oriented by the above treatment, the rubbing roller 1 must be rotated at a high speed. For example, the roller, when having a diameter of 10 cm, is rotated usually at a speed of 300 to 500 r.p.m.
Nevertheless, if treated at such a high speed of rotation, the surface of the synthetic resin layer is electrostatically charged greatly, while the rubbing fabric releases a large quantity of flock, with the result that removed pieces of flock, resin cut fragments and like extraneous fragments adhere to the orientation film and are brought to the subsequent process, consequently lowering the reliability of the liquid crystal display device.
It appears favorable to reduce the speed of rotation of the rubbing roller to obviate the drawback, whereas difficulty is then encountered in affording a film with a uniform desirable orientation.
The present invention, which has been accomplished in view of the foregoing problem, provides a process for preparing an orientation film for liquid crystals which is given a satisfactory orientation although treated with a rubbing roller at a greatly reduced speed of rotation.